|
Section 4: Rock mass assessment |
1 Department of Earth Sciences, University of Liverpool, Brownlow Street, Liverpool L69 3BX, UK
2 Golder Associates (UK) Ltd, 54 Moorbridge Rd, Maidenhead, Berkshire SL6 8BN, UK
To determine whether 130 felt earth tremors around Edwinstowe, Nottinghamshire, UK, which also experienced severe surface fissuring, were caused by coal extraction, a surface seismometer array was established around Thoresby Colliery. Over the next year, 785 microseismic events were detected. The spatial and temporal variations in seismicity are clearly associated with the commencement, continuing extraction and closure of faces. Of particular note are events which locate at the surface and appear to be related to the active fissuring. Events occur within days of commencement of production and cease when production finishes, with good correlation between face advance and hypocentral position. Naturally occurring microseismic events have also been detected up to 1 km ahead of active longwall faces in the Midlands using triaxial geophone packages grouted into the seam together with a surface seismometer in the top of the borehole. The quality of these data was very high and guided waves can clearly be seen with the dispersive characteristics associated with seam waves. In one experiment, more than 2000 events were detected in only two days of monitoring even in a relatively noisy surface environment. This paper demonstrates how useful locations can be generated using three-component digital data from only one borehole. The event distributions give a dynamic, three-dimensional image of the developing patterns of fracturing above, below and ahead of the longwall face with important implications for roof control, subsidence prediction and gas migration.
